Recording elements comprising write-once thin film alloy layers

ABSTRACT

Recording and record elements are disclosed. The elements have thin film optical recording layers of a SbSnGe alloy. The alloy has a composition within a polygon ABCDEF in a ternary SbSnGe composition diagram as shown in FIG. 5 herein.

FIELD OF THE INVENTION

This invention relates to recording elements and recording methods.

BACKGROUND OF THE INVENTION

Thin film optical recording layers using chalcogenide thin-films andamorphous to crystalline phase transitions have been the subject of manyinvestigations since the early 1970's. The initial interests werefocused on "erasable", and therefore reusable, optical recording layerssince the amorphous to crystalline transition is, in principle, areversible process. Such layers are generally prepared by a vacuumprocess. The layer is amorphous when so prepared. A low power,relatively long duration laser pulse is used to heat a local spot on thelayer to below the melting point for a sufficient length of time tocause the spot to crystallize. These crystalline spots can in turn beheated, by a higher power, shorter duration laser, above the meltingpoint of the crystallized spots to randomize the structure of the spots.The layer is designed such that upon the termination of the laser pulsethe cooling rate of the heated spot is high enough that the randomizedstructure is frozen to achieve an amorphous state.

Thus by adjusting the laser power and duration, the state of a selectedarea on the layer can be switched between the amorphous state and thecrystalline state to create a pattern of amorphous and crystalline spotswhich can be used for information storage. Since the phase transition isreversible, the pattern can be erased and replaced with a differentrecorded pattern. Theoretically, this erase-write cycle can be carriedout any number of times.

A principal difficulty is that the rate of crystallization of mostlayers studied is usually too low. For practical applications, it isdesirable to have layers which can be crystallized by laser pulsesshorter than a microsecond (μs). Presently, few materials havedemonstrated such capabilities. For some materials with highcrystallization rates (e.g. Te-Sn alloy), the data retention times areoften not adequate because of the instability of the amorphous state.

Because of the slow crystallization of most materials, thecrystallization step is generally used as the erasure step in erasableoptical recording layers. A laser spot elongated in the direction of thelaser movement is used to give an effectively long duration laserexposure. Such long laser spots cannot be used for high densityrecordings. The amorphizing step, on the other hand, is used as therecording step since this can be achieved with short laser pulse, andhence can be done at high speed.

Very few materials are known for optical recording layers in which theabove described write-erase-write cycle is of practical use. No erasablephase-change type optical recording layers have been commercialized.

A good deal of attention has also focused on so-called "write-once" thinfilm optical recording layers. Write-once simply means that the layerscan be recorded upon only once. Such layers cannot be erased and reusedfor a subsequent recording.

Since thin film optical recording layers are generally amorphous whenprepared, it is desirable to use the crystallization step as therecording step in write-once layers. However, the problem of slowcrystallization prevents the achievement of high data rates. High datarates are critical for write-once layers designed for use withcomputers.

European patent application No. 0184452 broadly discloses erasableoptical recording layers of antimony and germanium. No instructions aregiven regarding what the relative proportion of each element should bein the layers. Also no examples are given of antimony and germaniumlayers. Information recording and erasure are said to be achieved byswitching the layers between two different crystalline states. Thelayers are generally prepared in the amorphous states which have to befirst converted into one of the two crystalline states beforeinformation can be recorded. The crystallization step, achieved byeither a bulk heat-treatment or a prolonged laser exposure, is said tohave a lower reflectance than the amorphous state. Examples of antimonyand germanium alloys were not given and the examples of alloys otherthan antimony and germanium are disclosed. Layers of such alloys have avery low rate of crystallization. This application further teaches thatthe optical recording layers disclosed therein are unsuitable for use inthe amorphous-to-crystalline transition mechanism because of theinstability of the amorphous state in general.

Experimental evidence has shown that the crystalline-crystallinerecordings and the fast amorphous-to-crystalline recordings are mutuallyexclusive. Compositions which demonstrate properties suitable for onemode of recording are not suitable for the other mode of recording.

Another problem is that many of the chalcogen containing materials whichundergo the amorphous-to-crystalline transition mechanism are usuallycorrosion prone.

The problem is that the prior art has not provided write-once opticalrecording layers which possess the combination of (a) a crystallizationrate less than 1.0 μs, (b) good corrosion resistance, (c) a stableamorphous state and (d) a capability of high rate, high densityrecordings.

SUMMARY OF THE INVENTION

The present invention provides a recording element comprising awrite-once amorphous thin-film optical recording layer of an alloyhaving a composition within a polygon in a ternary composition diagramof antimony, tin and germanium described in FIG. 5 herein; wherein thepolygon has the following vertices and corresponding coordinates in atompercent:

    ______________________________________                                                 Coordinates                                                          Vertices   Sb          Sn       Ge                                            ______________________________________                                        A          86          13.99    0.01                                          B          55          44.99    0.01                                          C          18          52       30                                            D          18          42       40                                            E          78          0        22                                            F          98          0        2                                             ______________________________________                                    

The present invention also provides a record element having

(a) a composition within the above described polygon in FIG. 5; and

(b) a pattern of amorphous and crystalline areas in which thecrystalline areas are all in the same state with a higher reflectivitythan the amorphous state.

The elements of this invention do not suffer the environmental corrosionseen in chalcogen rich thin films. The rate of crystallization of theoptical recording layers is less than 1 μs using practical laser power.The amorphous state is very stable. Thus, recordings on the thin filmare made using the amorphous to crystalline transition mechanism. Thelayers are capable of high density, high rate recordings. Moreover thelayers cannot be switched between two different crystalline states assuggested by European patent application No. 0184452 and the crystallinestate is uniformly more reflective than the amorphous state.

It has been found that layers formed from alloy compositions outside ofthe defined polygon (a) are either crystalline as deposited or (b)crystallize too slowly to be of practical use. The layers have anamorphous to crystalline transition temperature of at least 80° C.

Layers used in the elements of the invention are capable of forming onlya single crystalline state. That is the crystalline state is the samethroughout the recorded layer. In many recordings the crystalline areaswill have a uniform composition.

Especially useful record and recording elements have alloy compositiionswithin the polygon in FIG. 5 having the following vertices andcorresponding coordinates:

    ______________________________________                                                 Coordinates                                                          Vertices   Sb           Sn     Ge                                             ______________________________________                                        A          86           13.99  0.01                                           B          55           44.99  0.01                                           C          18           52     30                                             D          18           42     40                                             I          75           2      23                                             J          96           2      2                                              ______________________________________                                    

Preferred record and recording elements have alloy compositions with thepolygon of FIG. 5 having the following vertices and correspondingcoordinates:

    ______________________________________                                                 Coordinates                                                          Vertices   Sb           Sn     Ge                                             ______________________________________                                        A          86           13.99  0.01                                           B          55           44.99  0.01                                           G          40           48     12                                             H          40           36     24                                             ______________________________________                                    

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a description of a schematic recording and readback apparatusfor using the recording elements of the invention; and

FIG. 2 is a schematic cross section of an optical recording element ofthis invention; and

FIGS. 3 and 4 are curves showing some of the experimental results ofexamples 1 and 3.

FIG. 5 is a ternary composition diagram showing polygons within whichuseful alloy mixtures of the present invention are found.

DETAILED DESCRIPTION OF THE INVENTION

Recording information on the thin film layers is achieved by focusing aninformation modulated laser beam on the layer thereby forming a patternof crystalline and amorphous areas on the layer. All the crystallineareas will be in the same state.

FIG. 1 shows a schematic of an apparatus for recording information on anoptical recording element 16 of the invention and for playing back therecorded information therefrom. Referring to FIG. 2, recording element16 comprises an overcoat layer 41, amorphous thin film optical recordinglayer 42 on substrate 45. In response to a drive signal, the intensityof a diode recording beam is modulated in accordance with information tobe recorded on thin film 42. The modulated laser beam is collected by alens 14 and collimated by a lens 18 and is directed by means of mirrorelements 20, 23 and 24 to a lens 26 which focuses the modulated laserbeam to a recording spot 28 on the film 42 as shown in FIG. 1.

During recording, the element 16 is spun at a constant rate, e.g. 1800rotations per minute (rpm). As a result, a track of information 30 isrecorded on the optical recording layer in the form of selectedcrystallized areas. As recording continues, the recording spot 28 iscaused (by means not shown) to scan radially inward across the element16, thereby causing information to be recorded along a spiral orconcentric track that extends from an outer radius r_(o) to an innerradius r_(i). The sizes and spacings of the recorded information marksvary in accordance with the information content of the recording laserdrive signal, as well as with radial position on the element 16.

During the readback process, the new information bearing element 16 isspun at the same rate as it was spun during the recording process. Alaser beam 22 from a readout laser is expanded in diameter by means oflenses 34 and 36. The optical path of the readout laser beam is foldedby a beam splitter 21 and mirrors 23 and 24 so that the readout laserbeam is focused to a playback spot on the element 16 by the highnumerical aperture lens 26. The element 16 is assumed to be of thereflective type so that the radiation forming the playback spot isreflected back through the high numerical aperture lens 26 afterinteracting with the information marks recorded on the optical element16. A lens 38 directs reflected laser radiation which has been divertedby the prism beamsplitter onto a detector 40 which produces anelectrical playback signal in response to temporal variations (contrast)in the irradiance of the reflected laser radiation falling on thedetector.

The amorphous thin film optical recording layers of this invention arewritten upon with a coherent beam of electromagnetic radiation ofsufficient energy to convert selected portions of the amorphous film 42to a crystalline state. In the present invention the amorphous thin filmoptical recording layers are of sufficient sensitivity that laser powersof about 2 to 10 mW at laser pulsewidth of 40 to 100 nanosecondsprovides sufficient energy to make the conversion.

Recordings on the amorphous thin film were made with a static pittester.

The static pit tester provides automated facilities in which amicrocomputer controls the sample position, the laser power and thelaser pulse-width. Each recording layer is exposed with a 830 nanometerlaser diode in the static pit tester to produce a matrix of spots inwhich the laser power is varied from 4 to 12 mW and the pulse widthvaried from 40 to 30,000 nanoseconds. The suitability of the recordinglayer for optical recording is determined by measuring the change inreflection between the exposed and unexposed areas of the sample, i.e.between the crystalline and amorphous states.

This reflection change is expressed as recording contrast, CT, by thefollowing definition: ##EQU1## wherein R_(c) and R.sub.α are thereflectances of the crystalline and the amorphous states, respectively.A minimum contrast of 5 percent must be achieved for the films to beconsidered useful as optical recording layers.

The thin amorphous film recording layers can be prepared by conventionalthin film deposition techniques such as evaporation, RF (radiofrequency) and DC (direct current) sputtering from an alloy target, andRF and DC co-sputtering from targets of the individual elements.Enhancement of sputtering processes by applying magnetic fields(magnetron sputtering) can also be used. The thickness of the films canbe from a few tens to a few hundreds nanometers depending on compromisesamong factors such as contrast, sensitivity, production rate, materialcost, ease of control, data rate, etc.

Supports which can be used include plastic films, such as polyethyleneterephthalate, polymethyl methacrylate, and polycarbonate, a glassplate, paper and metallic plates.

The practice of the invention is further described in the followingexamples. In the examples, each thin film optical recording layer isrepresented by the symbol Sb_(x) Ge_(y) Sn_(z) wherein x, y and z areatom percents.

EXAMPLE 1

Amorphous thin film optical recording layers of this invention wereprepared by a sputtering process. A target composed of homogeneouslymixed Sb and Ge powders was pre-sputtered in an 8 mtorr Ar atmospherefor one hour. The pre-sputtering step was designed to achieve a steadystate deposition condition.

Thin films of about 140 nm in thickness were then prepared by sputteringthe pre-sputtered mix for 7 minutes onto a glass support. The atomicfraction of each component in the prepared film was determined byinductively coupled plasma atomic emission spectrometry (ICP) and X-rayflorescence (XRF). The recording layer comprised 91.5% Sb and 8.5% Ge.The amorphous to crystalline transition temperature was 164° C. Thishigh transition temperature shows that the amorphous state of the filmsof the invention are very stable. This is an important keeping property.A very low transition temperature from amorphous to crystalline would bedetrimental to optical recording layers in that the reflectancedifference between written data encoded as crystalline marks andunmarked amorphous areas would be lost.

Another sample of the above film was written upon using the static pittester described herein before. The writing was in the form ofcrystallized marks on the films. The film with the crystallized writtenspots was placed in a chamber at 70° C. and 30 percent relative humidityfor an accelerated stability test. After 44 days, the film was examined.We did not observe any phase change or corrosion on the unwritten filmor the written spots. The film did not have any overcoat as a protectivelayer against corrosion. This test shows that the films of the inventionbearing written spots are also environmentally stable.

Another film sample of the same composition was subjected to performancetests on the static pit tester. The film was overcoated with a vacuumcoated 140 nm thick SiO₂ film to reduce deformation during the writingstep. A pulsed semiconductor laser beam with a wavelength of 830 nm wasused for writing. The writing sensitivity and contrast at various powersand pulse widths are shown in FIG. 3. FIG. 3 shows that the percentcontrast between the reflectance of the amorphous state and thereflectance of the crystallized state is clearly measurable and can thusbe read by state of the art laser read systems. These data also showthat the thin films can be written upon using practical laser powers andwriting speeds.

EXAMPLE 2

A number of amorphous Sb-Ge thin films with a range of compositions wereprepared according to the method in Example 1. Some of therepresentative compositions and their corresponding writingsensitivities (minimum required laser pulse length and power) are listedin the following: Sb₉₄ Ge₆, 50 ns, 6 mW; Sb₈₉ Ge₁₁, 100 ns, 6 mW; Sb₈₆Ge₁₄, 200 ns, 8 mW; Sb₈₄ Ge₁₆, 400 ns 8 mW; Sb₇₉ Ge₂₁, 1 μs, 10 mW.

The thin films of examples 1 and 2 are sensitive write-once opticalrecording layers. The films cannot be cycled between two differentcrystalline states as taught in European patent application No. 0184452.

EXAMPLE 3

Amorphous thin film optical recording layers of this invention wereprepared by the sputtering process of example 1.

Thin films of about 100 nm in thickness were then prepared by sputteringthe pre-sputtered mix for 4 minutes onto a glass support. The atomicfraction of each component in the prepared film was determined by ICP.The composition of the film on an atom to atom basis was 64% Sb, 30% Snand 6% Ge.

The amorphous to crystalline transition temperature was 152° C. as shownin FIG. 4. The heating rate was 25 milli-Kelvin per second.

This high transition temperature shows that the amorphous state of thefilms of this invention are very stable.

Another sample of the above thin film was written upon using the staticpit tester described herein before. The writing was in the form ofcrystallized marks on the film. The film with the crystallized writtenspots was placed in a chamber at 70° C. and 30 percent relative humidityfor an accelerated stability test. After 24 days, the film was examined.We did not observe any phase change or corrosion on the unwritten filmor the written spots. This test shows that the films of the inventionbearing written spots are also environmentally stable.

Another film sample of the same composition was subjected to performancetests on a static pit tester. A pulsed semiconductor laser beam with awavelength of 830 nm was used for writing. The resulting data showedthat the thin films can be written upon using practical laser powers andwriting speeds. The writing contrast was about 20 percent at a pulsewidth of 100 ns and 10 mW of laser power. The sensitivity of the filmwas such that it could be written upon at 40 ns and 4 mW of power.

EXAMPLE 4

A number of amorphous Sb-Ge-Sn thin films with a range of compositionswere prepared according to the method of Example 1. Some of therepresentative compositions are Sb₈₁ Ge₅ Sn₁₄, Sb₇₈ Ge₈ Sn₁₄, Sb₇₂ Ge₄Sn₂₄, Sb₆₉ Ge₇ Sn₂₄, Sb₆₆ Ge₄ Sn₃₀ and Sb₆₅ Ge₉ Sn₂₆. These films can bewritten upon at a laser pulse length of 50 ns and power of 6 mW. Thewritten information was in the form of crystalline spots.

EXAMPLE 5

A homogeneous Sb-Ge-Sn alloy sputtering target was prepared by hotpressing. An amorphous thin film, with a composition of Sb₇₄ Ge₄ Sn₂₂,was prepared by sputtering. The film can be crystallized at a laserpulse length of 50 ns and power of 4 mW.

COMPARATIVE EXAMPLE

Thin films were prepared in which the alloy compositions were (1) Sb₄₀Sn₅₈ Ge₂ and (2) Sb₅₈ Sn₂ Ge₄₀. Thin film (1) was crystalline whendeposited. Thin film (2) was amorphous when deposited but extremelydifficult to crystallize. Both of these films are outside of the scopeof the present invention.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modificatiions can be effected within the spirit and scope of theinvention.

We claim:
 1. A recording element comprising a write-once amorphousthin-film optical recording layer of an alloy having a compositionwithin a polygon in a ternary composition diagram of antimony, germaniumand tin; wherein(a) the composition diagram is ##STR1## and (b) thepolygon has the following vertices and corresponding coordinates in atompercent:

    ______________________________________                                                 Coordinates                                                          Vertices   Sb          Sn       Ge                                            ______________________________________                                        A          86          13.99    0.01                                          B          55          44.99    0.01                                          C          18          52       30                                            D          18          42       40                                            E          78          0        22                                            F          98          0        2                                             ______________________________________                                    


2. A record element comprising an optical record in a layer of an alloyhaving(a) a composition within a polygon in a ternary compositiondiagram of antimony, germanium and tin, wherein(i) the compositiondiagram is ##STR2## and (ii) the polygon has the following vertices andcorresponding coordinates in atom percent:

    ______________________________________                                                 Coordinates                                                          Vertices   Sb           Sn     Ge                                             ______________________________________                                        A          86           13.99  0.01                                           B          55           44.99  0.01                                           C          18           52     30                                             D          18           42     40                                             E          78           0      22                                             F          98           0      2;                                             ______________________________________                                    

and (b) a pattern of amorphous and crystalline areas which crystallineareas are all in the same state with a higher reflectivity than theamorphous state.
 3. An element according to claim 1 or 2 wherein thealloy has a composition within a polygon in a ternary compositiondiagram of antimony, tin and germanium, wherein:(i) the compositiondiagram is ##STR3## and (ii) the polygon has the following vertices andcorresponding coordinates:

    ______________________________________                                                 Coordinates                                                          Vertices   Sb           Sn     Ge                                             ______________________________________                                        A          86           13.99  0.01                                           B          55           44.99  0.01                                           C          18           52     30                                             D          18           42     40                                             I          75           2      23                                             J          96           2      2                                              ______________________________________                                    


4. An element according to claim 1 or 2 wherein the alloy has acomposition within a polygon in a ternary composition diagram ofantimony, tin and germanium, wherein:(i) the composition diagram is##STR4## (ii) the polygon has the following vertices and correspondingcoordinates:

    ______________________________________                                                 Coordinates                                                          Vertices   Sb           Sn     Ge                                             ______________________________________                                        A          86           13.99  0.01                                           B          55           44.99  0.01                                           G          40           48     12                                             H          40           36     24                                             ______________________________________                                    


5. The element of claim 1 or 2 having an amorphous to crystallinetransition temperature of at least 80° C.
 6. The element of claim 1 or 2in which the layer is capable of exhibiting only a single crystallinestate.
 7. The element of claim 1 or 2 in which the layer is capable ofexhibiting only a single crystalline state having a substantiallyuniform composition.
 8. The element of claim 1 or 2 in which the alloyhas the composition Sb₈₁ Ge₅ Sn₁₄, Sb₇₈ Ge₈ Sn₁₄, Sb₇₄ Ge₄ Sn₂₂, Sb₆₉Ge₇ Sn₂₄ or Sb₆₆ Ge₄ Sn₃₀.